My friend Fraser Cain, from Universe Today, put together a video chat Q&A about the discovery with me, Nancy Atkinson from UT, Emily Lakdawalla from the Planetary Society Blog, and Alan Boyle from the MSNBC Cosmic Log blog. We talked about the discovery, how it was made, what it means for exoplanetary science, and a few other topics just for good measure.

Fraser did this using Google+ Hangout, the social network’s video chat software. They rolled out a new feature just a few days ago where a few people can chat on camera, and the whole thing can be broadcast on G+ at the same time. Not only that, but, obviously, it can be recorded and uploaded to YouTube as well. This is brand new stuff, and not widely available just yet, so we had some issues with it (notably Fraser’s window never was displayed on the main screen; the images he displays at 13 minutes in can be found on the Kepler website).

Since I’ve got you here, there’s one very cool thing I’d like to expand on. Later in the video, we chatted about the physical characteristics of the Kepler-20 system, including how the planets’ orbits were tilted, and how you can determine that from the Kepler data. I poked around on the web afterwards, and found that the Kepler site has an amazing feature; an interactive display of all the confirmed planets they’ve found. For example, here’s the one for Kepler-20-f, the outermost of the five planets in the system, and the one closest to the size of Earth.

You can watch an animation of it going around the star, with a display of how it blocks the light. You can also see how the orbit is slightly tilted to the line of sight, and how it cuts a chord across the star. It’s truly a splendid way to show folks what they’ve found, and I highly suggest playing around with it (though it may be slow due to heavy use right now). When it loads, click the button labeled "Perspective" and then click "go to view from Earth". That’ll show you how a transit works pretty well.

From those pages, I found that these planets do orbit their star almost — but not quite — edge-on. An orbital inclination of 90° would be edge-on, and the planets, in order from the star, have tilts of 86.5, 88.4, 89.6, 87.5, and 88.7°. I was surprised to see that there is a spread of even as much as 3°. I wonder why? The planets probably formed farther out and migrated in toward the star; we know planetary migration happens for many (if not all) solar systems when they’re young, including our own. As these planets got closer, they could interact more strongly via gravity. Maybe that amplified their tilts somewhat. Or maybe I’m totally wrong in thinking the tilts should all be aligned in the first place.

We’re still new at this game, so there’s a lot left to learn. But that, my friends, is where the fun is. May we have lots more fun systems like Kepler-20 to investigate.

Astronomers have achieved a big milestone in the search for another Earth: the two smallest confirmed planets ever found orbiting another star… and they’re both about the size of Earth!

Artist’s illustration of the Kepler-20 planets with Earth and Venus for size comparison.

The planets are called Kepler-20e and Kepler-20f, and as you can see by the illustration above they are very close to the same size as our home world: 20e is about 11,100 km (6900 miles) in diameter, and 20f about 13,200 km (8200 miles) across. For comparison, Earth has a diameter of 12,760 km (7930 miles). This makes them the smallest confirmed exoplanets seen orbiting another star! The previous record holder was Kepler-10b, which has a diameter about 40% bigger than Earth’s.

To be clear: while these planets are the size of Earth, they are nowhere near Earth-like. The star, Kepler-20, is very much like the Sun, though a bit smaller and cooler (and 950 light years away). However, both planets orbit the star much closer than Earth does; 7.6 million km (4.7 million miles) and 16.6 million km (10.3 million miles), respectively. This is so much closer that both planets must have surface temperatures far hotter than ours, 760°C and 430°C (1400°F and 800°F). Even on the "cooler" planet Kepler-20f, it’s hot enough to melt tin and zinc.

So don’t start packing your bags to visit, even if you could spare a few million years to get there via rocket (950 light years is a bit of a hike). I’ll note that we don’t know the masses of these planets either. I’ll explain that in a moment, but given their sizes it’s expected they’ll have masses similar to Earth’s.

So this is very exciting! For one thing, it shows that Kepler can indeed find planets the size of Earth orbiting distant stars. That right away is fantastic; that’s the main goal of Kepler in the first place.

For another, it shows that our solar system is not entirely unique. We do know of several other stars hosting solar systems of their own, but those planets tend to be very massive; they’re easier for us to find. Since Kepler-20e and f are so close to Earth-sized, this is a big achievement.

And we’re still not done: there are three other planets in the Kepler-20 system! Read More